3n 



top in a thin sheet. In some cases, however, in- 

 stead of the water pouring over the entire length 

 of the dam the beaver force it to pour over in a 

 given stretch at one end or the other, or some- 

 times through a hole or tunnel. The concentra- 

 tion of the overflow at some one point in the dam 

 is commonly done either for the purpose of using 

 it in transportation or to force the water to out- 

 pour on a spot where it will least erode the 

 foundation of the dam. Occasionally beaver com- 

 pel the water to flow round the end of a dam, 

 which they raise sufficiently high for that purpose. 

 Sometimes they dig a waste-way for the water. 



European beaver appear to have barely devel- 

 oped to the dam-building stage. Rarely did they 

 build even a small, unimportant dam. Nor did 

 all the American beaver build dams. At the time 

 the beaver population was most numerous and 

 widely distributed, probably not more than half 

 of them used the dam. However, those not using 

 the dam were living in places where the dam and 

 consequent pond were not needed. Dam-building 

 enormously increased the habitable beaver area. 

 There were, and are, thousands of brooks which 



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